The present invention relates to a gas discharge tube to be principally used as a light source in analytical and quantitative measurements. The present invention also relates to an indirectly heated cathode for use with said gas discharge tube and a drive circuit for driving those.
A deuterium lamp as a gas discharge tube is shown in FIG. 5. The deuterium lamp generally indicated by 1 comprises a transparent sealed envelope 13 in which are housed an anode 11, a cathode 2 and a shield electrode 12. A small hole 22 for converging electrons and a window 23 through which light is transmitted are formed in the shield electrode 12. When a voltage is applied between the anode 11 and the cathode 2 with the latter being heated, an arc discharge is produced between the anode 11 and cathode 2 via the small hole 22. Only part of the anode light can be transmitted through the small hole 22 and then passes through the window 23, so that the small hole 22 acts as a point light source which emits high intensity light.
An indirectly heated cathode that may be used advantageously in the deuterium lamp 1 is described in Japanese Patent Application Examined Publication No. Sho. 62-56628. As shown in FIG. 3, this cathode comprises a cylinder 24 that is made of a heat-resistant and highly heat conductive material such as molybdenum and which is surrounded on its outer wall with a double coil 25 that is formed by winding a tungsten wire filament into a primary coil, which in turn is wound spirally into a secondary coil. A carbonate of barium, strontium or calcium that is in either an elemental or mixed form is applied both between turns of the primary coil and between turns of the secondary coil. A heater 3 in coil form is provided in the interior of the cylinder 24 which is mounted in the discharge tube by means of a support 21. The cylinder 24 is in electric connection with the heater 3 through the support 21. When an electric current is applied to the heater 3 with the discharge tube held in vacuum (.ltoreq.10.sup.-3 Torr), the carbonate undergoes a thermal decomposition reaction to form an electron emitting material 26 made of an oxide.
FIG. 2 shows a trigger type drive circuit which is most commonly used to drive gas discharge tubes. The heater 3 is constantly supplied with power from a heater power source 4 for a preheating purpose. After preheating for 10-60 seconds, a trigger switch 5 is changed over from a normally closed contact 6 to a normally open contact 7 and the electric charge stored in a capacitor 8 is discharged to light the discharge tube 1. However, the power consumption of the indirectly heated cathode 2 is so large that in order to insure stable operation of the indirectly heated cathode 2 in the gas discharge tube 1 during discharging, power must be continuously supplied from the heater power source 4 even after the discharge tube lit up.
As described above, the indirectly heated cathode 2 used in the conventional deuterium discharge tube 1 has had to employ the heater power source 4 which constantly supplies said cathode with power during discharging so that it works as an effective hot cathode that maintains a stable arc discharge. In other words, the conventional indirectly heated cathode 2 has suffered the disadvantage of consuming large power.